As a digital radio signal transmission system, an orthogonal frequency division multiplex (OFDM) system is known.
With this orthogonal frequency division multiplex system, a digital signal is multiplexed by using a number of subcarriers having different frequencies to transmit the digital signal efficiently. With the orthogonal frequency division multiplex, a guard interval section is disposed in front of an effective symbol section so that the reception characteristics can be prevented from being degraded by inter-symbol interference in a multipath environment.
An antenna switching type diversity receiver such as shown in FIG. 2 is known which can mitigate the influence of fading during mobile reception of radio signals subjected to orthogonal frequency division multiplex.
In a conventional diversity receiver shown in FIG. 2, signals received by respective antennas 15 to 18 are amplified by LNSs 19 to 22 and input to a selector 23. In response to a select signal supplied from a demodulator 25, the selector 23 selects one received signal and inputs it to a tuner 24. The tuner 24 frequency-converts a radio frequency (RF) in a desired band into an intermediate frequency (IF) signal and sends it to a demodulator 25.
The tuner 24 detects a signal level of the received signal and notifies the signal level to the demodulator 25 by using a received signal strength indication (RSSI) signal. The demodulator 25 monitors the RSSI signal, and when the signal level becomes a predetermined threshold value or lower, the demodulator 25 controls the selector 23 to select another received signal. If the signal level of the selected received signal is also the threshold value or lower, another received signal is again selected. This operation is repeated.
The conventional diversity receiver cannot detect in advance whether the signal level of a selected received signal becomes higher or lower than that of the presently selected received signal. Therefore, the reception state after selection may become worse than that before selection so that the receiver cannot stably receive signals subjected to orthogonal frequency division multiplex.
If the demodulator 25 shown in FIG. 2 demodulates a digital signal by using an equalizer, the frequency characteristics change abruptly when the received signal is selected. Therefore, equalization errors of the received signal become large, sometimes resulting in a high possibility of demodulation errors.
The conventional diversity receiver requires a circuit for controlling the operation of the selector 23 shown in FIG. 2 to select the received signal, posing a problem of a complicated circuit structure.
The invention has been made in consideration of the above-described circumstance. It is an object of the invention to provide a diversity receiver capable of reliably receiving signals subjected to orthogonal frequency division multiplex, by using a simple structure.